America is set to be hit by another blast from the polar vortex – although temperatures are likely to be warmer than last week’s extreme conditions.

The polar plunge is expected to move south from Canada, bringing colder air and sub-zero temperatures to the US this week.

Forecasters say it will sweep over the lower Mississippi Valley and Midwest on Tuesday and Wednesday, and then hit the East on Thursday.

The main thrust of the cold air will follow up a couple of days later.

However, the conditions will be far less challenging than last week’s record-breaking polar air blast which affected more than half the US population.

“Following the retreat of Arctic air this weekend, waves of progressively colder air will move southward over Canada this week,” said Paul Pastelok, AccuWeather.com’s lead long-range forecaster.

“We will likely see a piece of the polar vortex break off and set up just north of the Great Lakes spanning January 16 to 20.

“This next main arctic blast will not rival, nor will be as extensive as the event last week.”

Many areas are still recovering from last week’s polar vortex, which saw the mercury plunge to -12C (11F) in New York City and -24C in Chicago.

Ice has blocked the Delaware River in New Jersey, forcing roads to close and causing a severe flood risk.

Water has built up behind the ice dam, and emergency services fear when temperatures rise it could devastate the area.

“If in fact the ice jam breaks then we’ll have a rush of water which could cause flash flooding,” said Qareeb Bashir, from Trenton Emergency Management.

A Coast Guard cutter was brought in to keep shipping lanes open, but the ice was too thick to break in places.

Residents have flocked to the river banks to take pictures of the polar conditions.

Rick Wilson, from Yardley, Pennsylvania, told an ABC TV station: “Incredible. I came down here just to take pictures of this. My grandchildren would not believe this. This looks like something you’d find in Antarctica.”

Sky News weather producer Jo Robinson said: “After a milder spell, plunges of cold air are expected later in the week.

“The first is expected across parts of Canada, the Midwest and eastern parts of the US over the next few days.

“More significant cold air will affect those areas by the weekend, but thankfully it doesn’t look to be as cold as last week.”